Disk drives, also called disk files, are information storage devices that use a rotatable disk with concentric data tracks containing the information, a head or transducer for reading and/or writing data onto the various tracks, and an actuator connected to a carrier for the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The actuator is a voice coil motor (VCM) comprising a coil movable through a magnetic field generated by a fixed permanent magnet assembly. There are typically a plurality of disks separated by spacer rings and stacked on a hub that is rotated by a disk drive motor, also called a spindle motor. A housing supports the spindle motor and head actuator, and surrounds the head and disk to provide a substantially sealed environment for the head-disk interface.
In conventional magnetic recording disk drives, the head carrier is an air-bearing slider that rides on a bearing of air above the disk surface when the disk is rotating at its operational speed. The slider is maintained next to the disk surface by a suspension that connects the slider to the actuator. The slider is either biased toward the disk surface by a small spring force from the suspension, or is "self-loaded" to the disk surface by means of a "negative-pressure" air-bearing surface on the slider. In contrast to conventional air-bearing disk drives, contact or near-contact disk drives have been proposed that place the head carrier in constant or occasional contact with the disk or a liquid film on the disk during read and write operations. Examples of these types of disk drives are described in IBM's U.S. Pat. No. 5,202,803 and published European application EP 367510; U.S. Pat. No. 5,097,368, assigned to Conner Peripherals; and U.S. Pat. No. 5,041,932, assigned to Censtor Corporation.
Most conventional magnetic recording disk drives are of the contact start/stop (CSS) type that operate with the slider in contact with the disk surface during start and stop operations when there is insufficient disk rotational speed to maintain the air bearing. To minimize the effect of "stiction", i.e., the static friction and adhesion forces between the very smooth disk surface and the slider, CSS disk drives often use a dedicated "landing zone" where the slider is parked when the drive is not operating. The landing zone is typically a specially textured nondata region of the disk.
In contrast to CSS disk drives, "load/unload" disk drives address the stiction problem by mechanically unloading the slider from the disk when the power is turned off, and then loading the slider back to the disk when the disk has reached a speed sufficient to generate the air bearing. The loading and unloading is typically done by means of a ramp that contacts the suspension when the actuator is moved away from the data region of the disk. The slider is thus parked off the disk surface with the suspension supported in a recess of the ramp. Load/unload disk drives provide a benefit in laptop and notebook computers because the parking of the slider on the ramp away from the disk surface also provides some resistance to external shocks caused by moving or dropping the computer.
The parking of the sliders on the load/unload ramp during disk drive power down is typically accomplished by use of the back electromotive force (EMF) generated by the freely rotating spindle motor. When the disk drive supply voltage is removed, the VCM is disconnected from its driver circuitry and connected to a rectifier circuit that is coupled to the spindle motor. The output of the freely rotating spindle motor is converted by the rectifier circuit to a DC current supplied to the coil of the VCM. This causes the actuator to move the sliders to the ramp. A significant amount of torque is needed to ensure that the sliders are fully parked on the ramp, regardless of the actuator position or velocity at power down. If a drive fails to unload the heads from the disk surfaces before spinning down, an unrecoverable stiction failure may occur. A three-phase, full-wave rectifier circuit with Schottky barrier diodes is commonly used for this purpose. Because there are always two diodes in series with the VCM load, the total voltage drop in this rectifier circuit can be relatively high, thus reducing the DC current available to the VCM. The diodes can be replaced with field-effect transistors (FETs) to reduce the voltage drop, but such devices need to be switched on and off synchronously with each phase of the spindle motor. This requires additional sensing and control circuitry which needs its own DC power source, e.g., a storage capacitor, because the back EMF is insufficient to also power this circuitry. IBM's U.S. Pat. No. 5,486,957 describes a high-efficiency, low-cost actuator retract rectifier circuit that uses bipolar transistors that are turned on by a small amount of current from the spindle motor, with the remaining spindle motor current being directed through the emitter-collector paths of the transistors to the coil of the actuator. Since the initial conditions of actuator position and velocity are unknown at the moment of power-off, sufficient torque must be applied to handle the worst case situation, i.e., actuator at the outside diameter (OD) of the disk near the load/unload ramp with zero velocity. However, if the retract system applies immediately the full torque needed for the worst case, excessive actuator acceleration and velocity may occur, resulting in excessive impact as the actuator hits the OD crash stop when it reaches the parking ramp. An example of when excessive acceleration occurs is the following: If the actuator is at rest near the inside diameter (ID) of the disk, or moving toward the ID from any location on the disk, the applied retract torque will first move the sliders across the disks (from ID to OD) before reaching the ramp. During this time, the lack of resistance to actuator motion results in large acceleration and velocity before reaching the load/unload ramp. Excessive impact occurs at the crash stop. High-seed camera investigation has shown about a factor of 2X actuator speed difference at the ramp between the cases of the actuator starting at rest at the ID or at rest at the OD. Excessive impact is undesirable because it can damage the fragile head/suspension assemblies, and cause significant dynamic pitching and rolling of the sliders when the actuator hits the crash stop. Excessive slider motion can cause the sliders to contact the ramp structure, or perhaps other sliders. Such contact can result in slider damage or transfer of contamination to the air-bearing surface, which can lead to head-disk interface failures.
What is needed is a simple, low-cost, power-efficient rectifier circuit that provides a high enough torque to bring the actuator to the disk OD under all initial conditions without excessive impact of the crash stop, but yet sufficient to properly park the sliders on the load/unload ramp.